1. Field of the Invention
The present invention relates to an optical pickup servo-control apparatus for use with an optical disc player.
2. Description of the Related Art
Servo control is usually used to control an optical pickup for reading recorded information at a desired reading position for the purpose of achieving good reproduction performance in a player for optical discs, such as compact discs (CD) and digital versatile discs (DVD). An optical disc, however, usually contains defects, such as various kinds of flaws and contamination which occur during, for example, the manufacturing process or the use of the disc. Such defects impede a stable operation of the above-mentioned servo control.
FIG. 1 is a sectional view of an optical disc 1, showing the kinds of major defects which may occur. The optical disc 1 includes a protection layer 3, a reflective layer 4 which reflects the light beam emitted from the optical pickup, and a transparent cover layer 5 made of a transparent material such as a plastic and the like.
Referring to FIG. 1, a defect I is a flaw called an “interruption” in the reflective layer 4, caused in the manufacturing process of the disc. A defect II is a dirt called a “black dot” on the disc's surface or the transparent cover layer 5. A defect III is a “fingerprint” of human fat on the disc surface 7. A defect IV is a flaw called a “scratch” on the disc surface 7.
The existence of such defects on a disc causes “dropout” in read signal (RF signal) generated in the optical pickup when the disc is played, and deteriorates the controllability of servo control including tracking control, focusing control, tilt control and the like. A conventional method, for example, for preventing the adverse influence on servo control caused by such defect is to continue servo control by holding the control value of servo control, or the tracking error value, focus error value, or the like to the value before the defect was detected (hereinafter, simply referred to as “pre-value hold”).
The servo control, however, becomes unstable when the dropout continues for a relatively long time or the dropout is of a burst type, because a bias or deviation value indicated by a value in the pre-value-hold method (hereinafter, simply referred to as a “hold value”) drifts too far from the actual bias value. For example, the drive signal for the optical pickup drastically fluctuates during the period when a dropout occurs, causing servo control instability as shown in FIG. 2. In addition, as the transient phenomenon arises after termination of the dropout, servo control becomes unstable because the focus error signal is delayed in converging into a normal value, for example, as shown in FIG. 3. Therefore, the performance of servo control may be deteriorated or a gross deterioration in reproduction quality may be caused.
The defect such as a fingerprint or a black dot is usually formed over a plurality of tracks in the radial direction of the disc (i.e., in a certain range of a rotational angle of the disc). Servo control also needs to be stabilized effectively against such defects.